Set CD/DVD drive first to boot in BIOS setup, place Repair CD in drive, restart computer and look for prompt to Press Any Key to boot CD.

The Repair CD should boot if the computer will start and is clear of RAM problems as stated, even if the HD is bad.

You can test the HD using maker's diagnostics/repair extended CD scan which will autostart: HD Diagnostic

It's possible that the BIOS has become infected so try booting Microsoft Standalone System Sweeper from USB flash stick or CD to scan HD. Knowing if the HD is infected badly can help determine if the BIOS is likewise infected, for which you may need to flash the BIOS with latest update.

I would also try another know-good HD to see what happens, or remove the existing HD from boot order to see if the Repair CD will boot.

If there is a factory Recovery partition on the HD you can also try booting it to run Recovery using the hotkey given on first boot screen or in Manual.